The present invention relates to a heater control apparatus and a heater control method of an air-fuel ratio sensor which is mounted to an exhaust system in an internal combustion engine and equipped with a heater for heating a sensor element.
Heretofore, an air-fuel ratio control apparatus of an internal combustion engine is known that detects an actual air-fuel ratio based on the oxygen concentration in the exhaust and the like using an air-fuel ratio sensor, and feedback controls a fuel supply quantity to the engine so that the actual air-fuel ratio reaches a target air-fuel ratio.
In order to perform the above-mentioned air-fuel ratio feedback control, it is precondition that the air-fuel ratio sensor is already activated. Since the air-fuel ratio sensor is activated when the temperature of the element thereof reaches a predetermined activation temperature, as shown in Japanese Unexamined Patent Publication No. 11-264811, the air-fuel ratio sensor is equipped with a heater for heating the sensor element, thereby controlling the power supply to the heater after an engine start.
Specifically, the power supply to the heater is controlled after the engine start (duty-control). At first, the power supply is made in an initial duty value set based on a cooling water temperature at the engine start, and a power supply amount is increased gradually with the lapse of time so that a maximum duty value can be obtained within a predetermined control time after the engine start. The reason the power supply to the heater is increased gradually with the lapse of time is that the quick activation of the sensor element can be performed well while preventing a damage of the sensor element due to a heat shock.
On the other hand, Japanese Unexamined Patent Publication No. 61-122556 discloses that, for the purpose of controlling a power supply amount to a heater for heating a sensor element equipped in an air-fuel ratio sensor based on an element temperature, the element temperature is detected using an impedance of the sensor element since the impedance of the sensor element depends on the element temperature.
Specifically, an alternating voltage with high frequency is applied to the sensor element of the air-fuel ratio sensor, and the impedance of the sensor element is measured by a current value (amplitude) flowing in the sensor element caused by the application of the alternating voltage, thereby detecting the element temperature from the measured impedance.
In Japanese Unexamined Patent Publication No. 10-26599 is disclosed a feedback control of power supply amount to a heater so that an impedance of a sensor element reaches a target impedance.
In a heater control apparatus of an air-fuel ratio sensor, the exhaust performance is required to be improved in such a way that a power supply amount to the heater after an engine start is possibly made large, to raise rapidly an element temperature, and as a result, the quick activation of sensor element is achieved to promote the start of an air-fuel ratio feedback control. On the other hand, in a state where a wall temperature of an exhaust system is low, a water content in the exhaust discharged from the engine is condensed, that is, a state where the condensed water is generated, if the element temperature of the air-fuel ratio sensor rises up, the element is cracked by a heat shock when the condensed water is in contact with the sensor element. Accordingly, it is preferable that a rise of the element temperature is restrained until the water content exceeds a dew point.
Therefore, in a heater control apparatus disclosed in Japanese Unexamined Patent Publication No. 11-264811, a control to restrain the rise of element temperature is performed. However, since this heater control is a uniform control without monitoring an actual element temperature, in a case where there are variations in each sensor due to an element shape, a heater capacity, deterioration and the like, or variations in heater control circuit including a voltage fluctuation, and further in case where a condensation generation condition is varied due to variations of environmental conditions such as an atmospheric temperature, rain and the like, there causes a problem in that the element crack due to exhaust condensed water is unavoidable.
The present invention, in view of the foregoing problems, has an object of providing an apparatus and a method for controlling a heater in an air-fuel ratio sensor, which can avoid an element crack of the air-fuel ratio sensor due to exhaust condensed water when performing a heater control of the air-fuel ratio sensor.
Therefore, according to the present invention,
an element temperature of a sensor element of an air-fuel ratio sensor is detected by measuring an impedance of the sensor element,
a target temperature of the element temperature is set and a power supply amount to a heater is feedback controlled so that the element temperature reaches the target temperature, and
the target temperature of the element temperature is restrained to a lower temperature side, compared to other conditions, on a condition that a water content in the exhaust is condensed in an exhaust system.
According to this construction, the element temperature is detected by measuring an impedance of the sensor element of the air-fuel ratio sensor, and when the power supply amount to the heater is feedback controlled so that the element temperature reaches the target temperature, the target temperature is set at the lower temperature side on the condition that the water content is condensed. As a result, an element crack due to the exhaust condensed water can be avoided by maintaining the element temperature at a low temperature, and on the other conditions, the sensor element can be activated quickly by setting the target temperature at a higher temperature side. Further, since the feedback control is performed by detecting the element temperature by the impedance having a high correlation therewith, without an influence of variations in components the element crack due to the exhaust condensed water can be avoided certainly.
Further, according to the present invention,
an element temperature of a sensor element of an air-fuel ratio sensor is detected by measuring an impedance of the sensor element,
a target temperature of the element temperature is set and a power supply amount to a heater is feedback controlled so that the element temperature reaches the target temperature, and
the target temperature of the element temperature is restrained to a lower temperature side, compared to other conditions, on a condition that a water content in the exhaust is condensed in an exhaust system.
More specifically, an initial value of power supply amount to the heater is set corresponding to the element temperature before the start of power supply to the heater. The power supply amount to the heater is set to the initial value when the power supply to the heater is started. After starting the power supply to the heater, the power supply amount to the heater is feedback controlled so that the element temperature reaches a predetermined target temperature.
Here, the initial value of the power supply amount to the heater is set smaller as the element temperature before the start of power supply to the heater is lower.
According to this construction, the element temperature is accurately detected by measuring the impedance of the sensor element of the air-fuel ratio sensor, and when the power supply amount to the heater is feedback controlled so that the element temperature reaches the target temperature, the initial value of the power supply amount to the heater is set corresponding to the element temperature before the start of power supply to the heater. Thus, an element crack due to the exhaust condensed water can certainly be avoided by delaying the rise of the element temperature (small initial value) when the element temperature is low and the water content in the exhaust is likely to be condensed. On the contrary, the quick activation can be achieved by promoting the rise of the element temperature (large initial value) when the element temperature is high and the water content in the exhaust is unlikely to be condensed. Reexamination of the initial value constant caused by variations in components is not required and adaptation for each engine type is not necessary or is reduced by a large margin.
Further, according to the invention,
an element temperature of a sensor element of an air-fuel ratio sensor is detected by measuring an impedance of the sensor element,
an increase component of power supply amount to a heater is set corresponding to the detected element temperature, and
the power supply amount to heater is feedforward controlled so that the power supply amount to the heater is increased gradually by each increase component from a predetermined initial value.
Here, the increase component of the power supply amount to the heater is set smaller as the element temperature is lower.
According to this construction, when the power supply amount to the heater is feedforward controlled so that the power supply amount to the heater is increased gradually by each predetermined increase component from the predetermined initial value, the element temperature is accurately detected by measuring the impedance of the sensor element of air-fuel ratio sensor, and the increase component of the power supply amount to the heater is set corresponding to the detected element temperature. Thus, an element crack due to the exhaust condensed water can certainly be avoided by delaying the rise of the element temperature (small increase component) when the element temperature is low and the water content in the exhaust is likely to be condensed. On the contrary, the quick activation can be achieved by promoting the rise of the element temperature (large increase component) when the element temperature is high and the water content in the exhaust is unlikely to be condensed. Simplification of the control can be made by the use of feedforward control.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.